The present invention relates to an engine control method for a vehicle, and more particularly, to a method for reducing hydrocarbon emissions during cold start and engine idling.
Generally, a vehicle engine is controlled on the basis of the driving state, an amount of injected fuel, an amount of intake air, ignition timing, and the like. These control processes are performed according to a predetermined control logic by comparing signals of various sensors with data stored in an ECU (Electronic Control Unit).
A typical idle control method is illustrated in FIG. 2. According to such a method, if an ignition switch is turned on, the ECU detects an engine speed n and an engine load L and then calculates the idle speed actuator (ISA) open position therewith using a predetermined table. The ISA is opened to the calculated opening rate (S210). The engine speed n and the engine load L can be acquired from signals from the throttle position sensor and the crankshaft position sensor.
While the engine is operating under the calculated ISA open position, the ECU determines whether the engine speed is higher than a predetermined speed K (S220). If the engine speed is higher than the predetermined speed K, the procedure enters into an idle mode (S230). If the engine speed is not higher than the predetermined speed K, the procedure returns to step S210 where the ECU further regulates the ISA open position.
After step S230, the ECU calculates the air flow rate using the engine speed n and the coolant temperature T (S240). The ECU calculates the air/fuel ratio and the ignition timing using the engine speed n and the engine load L (S250 and S260). The ECU then determines whether the shift range is a neutral range or a parking range (S270). If it is determined that the shift range is the neutral range or the parking range, the ECU determines whether present engine speed is equal to a predetermined engine idle speed, the predetermined engine idle speed being determined by the coolant temperature T (S280).
If it is determined that the present engine speed is not equal to the predetermined engine speed in step S280, the procedure returns to step S240. If it is determined that the shift range is neither the neutral range nor the parking range but in a driving range in step S270, the ECU terminates the idle mode control and starts a D-range-mode control (S290).
In an engine control method as described above, feedback control is performed such that the engine speed approaches the predetermined idle speed. Engine speed is constantly monitored and controlled because as the engine warms up and friction resistance decreases, the initial ISA setting may cause the engine to idle too fast. There is, however, a limitation to the accurate control of the amount of air intake and idle speed using the ISA. Thus air/fuel ratio control and the ignition timing control are also used for more accurate engine control.
The air/fuel ratio control and ignition timing control are also changed because the amount of ISA air intake fluctuates and if the ignition timing is retarded during the cold idle state to decrease the time required to activate the catalytic converter, it is difficult to precisely control the engine. Emissions therefore increase. Further, as the engine speed increases, the ISA is controlled in such a manner that the amount of intake air is decreased. Therefore, it becomes difficult to decrease the time period for activating the catalytic converter.
The present invention provides an engine control method for decreasing hydrocarbon emissions during cold start and engine idling by retarding the ignition timing and controlling the engine to idle under a lean air/fuel ratio, and controlling the amount of injected fuel according to a suitable quantity of wetting fuel. In accordance with the present invention, the engine is cranked under a leaner air/fuel ratio than a stoichiometric air/fuel ratio by maximizing the amount of intake air in the early stage of the engine starting, and minimizing the engine load caused by an automatic transmission.
The quantity of wetting fuel is preferably calculated based on the number of power strokes of a specific cylinder and the total number of power strokes of the engine that have an effect on temperature of the specific cylinder. After starting, for a certain time period, idle speed is regulated by ignition timing in a state such that a maximum amount of intake air is secured.
In one embodiment, the engine control method according to the present invention comprises opening the ISA to a predetermined open position such that an air/fuel ratio becomes higher (leaner) than a stoichiometric air/fuel ratio, and minimizing engine load from the automatic transmission. The air/fuel ratio and ignition timing are controlled based on engine speed and engine load, and the amount of injected fuel is controlled based on the quantity of wetting fuel. The air flow rate is determined according to engine speed and coolant temperature. Air/fuel ratio and ignition timing are then controlled in order to control engine speed fluctuation based on engine speed and engine load.
According to a further preferred embodiment, the ISA is fully opened and the engine load due to the automatic transmission is minimized by controlling line pressure in the automatic transmission such that no line pressure is generated. Then the air/fuel ratio is set at a value higher than a stoichiometric air/fuel ratio, ignition timing control is performed, and the amount of fuel injected is controlled based on the quantity of wetting fuel that is determined in consideration of the number of power strokes in a specific cylinder and the total number of power strokes of a specific engine.
Air flow rate is calculated based on the engine speed and coolant temperature, simultaneously the ISA is fully opened and the air/fuel ratio and ignition timing are controlled based on engine speed and engine load. A series of determinations are made as to whether the engine speed is equal to a predetermined idle speed, whether the shift range is a neutral range or a parking range, if it is determined that the engine speed is not equal to the predetermined idle speed, and whether the elapsed time from the moment when the ignition switch is turned on is less than a predetermined time, if the shift range is the neutral range or the parking range.
The ISA is fully opened if it is determined that the elapsed time from the moment the ignition switch is turned on is less than the predetermined time. Additionally, it is determined whether a change in engine speed is greater than a predetermined critical speed change.
If the change in engine speed is greater than the predetermined critical speed change, engine speed fluctuation is controlled according to air/fuel ratio control and ignition timing control. If the change in engine speed is not greater than the predetermined critical speed change, the engine speed fluctuation is controlled according to ignition timing control alone.